When the peak-to-average power ratio of a transmission wave is relatively high in a wireless device that performs orthogonal modulation, the input signal level for a power amplifier has to be reduced. Accordingly, the power amplifier has to have a wide dynamic range. However, when the dynamic range of the power amplifier is increased, power consumption may be increased. Therefore, the amplitude of the input signal for the power amplifier has to be limited to reduce the peak power of the input signal.
FIGS. 9A and 9B are diagrams illustrating exemplary amplitude limiting operations according to the prior art. It is noted that in these drawings, the horizontal axis represents the I-component of an orthogonally modulated signal, and the vertical axis represents the Q-component of the orthogonally modulated signal. FIG. 9A illustrates a case where the amplitudes of the I-component and the Q-component of the orthogonally modulated signal are limited to be less than or equal to predetermined amplitudes (max). FIG. 9B illustrates a case in which the amplitudes of the I-component and the Q-component are limited by a circular boundary.
In FIG. 9A where the amplitude is limited by a square boundary, the amplitudes of the I-component and the Q-component are limited to be less than or equal to predetermined amplitudes (max). For example, the amplitude limiting operations may be described as follows:if(I>max)I=maxif(Q>max)Q=maxIt is noted that in the above-described operations, ‘I’ denotes the I-component, ‘Q’ denotes the Q-component, and ‘max’ denotes the limit value. As can be appreciated, these operations may be completed by executing one comparison process and one substitution process, and thereby, the amplitude limiting operations may be speedily executed by a simple hardware configuration.
In FIG. 9B where the amplitude is limited by a circular boundary, the peak value may be maintained to be less than or equal to a predetermined limit value (max). Accordingly, the peak value may be effectively suppressed to be prevented from flaring. For example, the corresponding operation may be described as follows:if(I*I+Q*Q>max)I=max*I/square root(I*I+Q*Q)It is noted that this operation requires multiplication, division, and squaring processes in addition to comparison and substitution processes. (see Patent Reference 1: Japanese Laid-Open Patent Publication No. 2003-168931)
It is noted that when a square limit range is used as in the case of FIG. 9A, although the operation processes and hardware configurations may be simplified, the amplitude may not be adequately limited when signal components are limited to be close to the peak point P31 in which case the peak value may not be adequately suppressed.
When a circular limit range is used as in the case of FIG. 9B, although the peak value may be effectively suppressed, multiplication, division, and squaring processes are required in addition to comparison and substitution processes so that the operational processes and the hardware configurations may be complicated and difficult to implement.